1. Field of the Invention
The present invention relates to the field of secondary batteries employing non-aqueous electrolytes which have good charge-discharge cycle characteristics and high discharge capacity, and more particularly to active materials for the negative electrode of secondary batteries employing lithium containing composite oxides such as lithium cobaltate (LiCoO2) and lithium nickelate (LiNiO2) for the positive electrode.
2. Description of the Prior Art
Typical active materials used for the negative electrode of non-aqueous secondary batteries (hereafter referred to as secondary batteries) are metallic lithium and lithium alloy. However, secondary batteries using metallic lithium as the negative electrode have not yet been put to practical use due to internal short-circuiting caused by dendritic growth of lithium on the negative electrode, or danger of fire by activation of the dendrite of lithium itself. A range of secondary batteries using lithium alloy have also been proposed, but these secondary batteries also have yet to be put into practical use due to the problem of disintegration into powder of the alloy electrodes during charge-discharge cycles, resulting in insufficient cycle reversibility. On the other hand, lithium ion secondary batteries using carbon materials as the negative electrode, such as graphite, which can reversibly intercalate and de-intercalate lithium, have been successfully put into practical use. The marketed type of lithium ion secondary battery employs the lithium containing composite oxide LiCoO2 for the positive electrode, and the lithium ions contained in the positive electrode at the beginning are reversibly intercalated and de-intercalated with the carbon material in the negative electrode during the charge-discharge cycles.
Compounds with this function other than LiCoO2, including LiNiO2, LiMn2O4, and their composite oxides, can also be used as the positive electrode in the lithium ion secondary battery. These compounds are suitable active materials having about +4 V higher electrode potential than that of metallic lithium; and they also feature a large reversible capacity, making high voltage and higher capacity feasible.
On the other hand, although the carbon material used for the negative electrode is regarded as having a large capacity, a theoretical charge-discharge capacity of 370 mAh/g is the upper limit even for the most commonly adopted graphite. The use of compounds other than carbon materials for the negative electrode, particularly combined with lithium containing composite oxide for the positive electrode, has the potential for increasing the capacity of lithium ion secondary batteries.
The use of metal oxides allows the realization of extremely high capacity, and an electrode material demonstrating a charge-discharge capacity greater than 1000 mAh/g has already been reported. Proposals made on the use of oxides for the negative electrode, which may promise high capacity, include tungsten oxide and lithium containing iron oxides (Japanese Patent Laid-open Publication No. H3-112070), niobium oxide (No. H2-82447), iron oxide and cobalt oxide (No. H3-291862), lithium containing silicon oxide (No. H6-325765), oxides containing vanadium (No. H7-14580), composite oxides containing tin, germanium or silicon (No. H7-201318), and amorphous oxides containing tin, lead, or silicon (No. H7-288123).
Recently, the use of lithium nitride metal compounds (lithium containing composite nitrides) has also been disclosed (No. H7-78609) as an electrode material for an electrochemical element.
However, most of these metal oxides show large irreversible capacity, although they have high capacity, when they are used for the negative electrode of the secondary battery. This disadvantage prevents them from being commercialized. Irreversible capacity is capacity loss and is caused by the capture of Li into crystals of the compounds. As a result, the capacity of the secondary battery decreases markedly because only a portion of the lithium supplied from the positive electrode to the negative electrode during the initial charge returns to the positive electrode during a subsequent discharge cycle.
Some of the means for counteracting this irreversible loss of capacity include the method of electrochemically treating the electrodes in advance to replenish irreversible lithium, and of replenishing the irreversible capacity by sticking metallic lithium to the negative electrode. For example, in case of the use of lithium containing silicon oxide as described above, silicon oxide is electrochemically treated in advance during the manufacturing process to include lithium in the oxide. The electrode electrochemical treatment method has an advantage that it is easy to control capacity by adjusting only the charging conditions. However, this method has a disadvantage of complexity and low productivity, because the secondary batteries must be reconstructed after electrodes are charged.
The method of sticking metallic lithium results in natural migration of Li between the oxide and the metallic lithium, both of which are short-circuited by a filling of electrolyte. However, this method has a disadvantage with respect to quality, including variations in secondary battery characteristics and safety due to remaining metallic lithium caused by insufficient migration of lithium, which depends on the shape of the electrodes.
These disadvantages prohibit commercialization of secondary batteries using oxides as the negative electrode although oxides are promising secondary battery materials. Accordingly, there is increasing demand for a simple technique to counteract irreversible capacity without risking secondary battery quality.
The present invention aims to offer a method for efficiently counteracting the irreversible capacity of a negative electrode employing compounds such as oxides.
The secondary battery of the present invention employs lithium containing composite oxide as a positive electrode material which can intercalate and de-intercalate lithium; and compounds with large irreversible capacity combined with lithium containing composite nitrides as a negative electrode material.
The secondary battery of the present invention further employs lithium containing composite oxides as the positive electrode material which can intercalate and de-intercalate lithium; and metal oxide as the negative electrode. The negative electrode also contains lithium containing composite nitride represented by the formula Li3xe2x88x92XMxN (M is a transition metal element, 0.2 less than xxe2x89xa60.8) in addition to the metal oxide. Furthermore, lithium containing composite nitrides including cobalt (Co) as a transition metal element M are preferable because they have high capacity and good reversibility.
For the positive electrode of the secondary battery of the present invention, lithium containing composite oxides such as lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), their compounds (LiCoxNiyO2, x+y=1) and lithium manganate (LiMn2O4) may be employed.